Great game series. CC5 has a great campaign, too bad it's so buggy. Basic advice:

When attacking, supress the target area with fire, move one group up, then leapfrog over. Never move into an area that you have not supressed first unless you know for certain that there are no enemies there.

Enemies in fortified positions should be flanked and assaulted from close range while being supressed from the front.

Always support your units!

Keep a couple units back as reinforcements in case something goes wrong.